This invention is directed to a method of process control. More particularly, this invention is directed to a method for effecting energy savings in relation to a refrigeration system of an ammonia production plant.
It is perhaps all too well known that ammonia production is currently an energy-intensive industry. It is also widely recognized that the cost of such energy, although not burgeoning at the rate of a decade ago, is currently a major portion of the total cost of ammonia production. Accordingly, small increases in ammonia production efficiency, effected by more efficient use of energy, can have significant impact in reducing ammonia production costs.
In certain ammonia production plant refrigeration systems (see, for example, Hydrocarbon Processing Magazine, published by Gulf Publishing Company, Vol. 60, No. 11, pp. 129-132, November, 1981), product ammonia is used as refrigerant in a refrigeration system of the ammonia production facility. The refrigeration system usually includes a condenser. Compressed product ammonia is introduced into the condenser and therein is cooled by a cooling medium, usually cooling-tower water.
Product ammonia usually contains dissolved non-condensible gases, such as nitrogen, methane, hydrogen, argon and helium, for example, which accumulate in the condenser as non-condensibles. Such non-condensibles are usually selectively purged from the condenser by a purge stream. The purge stream flow is usually adjusted to maintain a predetermined pressure in the condenser.
The temperature of the cooling medium being introduced into the condenser affects the temperature of the cooled product ammonia exiting the condenser. If the condensing temperature of the product ammonia remains fixed, and if the pressure within the condenser is held constant, the percentage of non-condensibles in the condenser will also remain constant. However, in a conventional ammonia production plant, the condensing temperature of the product ammonia does not remain fixed because of seasonal changes of the temperature of the cooling medium being supplied to the condenser.
Conventional ammonia production plant refrigeration systems use a pressure controller to maintain condenser pressure at a predetermined value. Such a predetermined pressure value may or may not be adjusted periodically between summer and winter conditions.
If the condensing temperature of the product ammonia changes, and if pressure in the condenser is held constant, the percentage of non-condensibles in the condenser will vary according to a well-known relationship, wherein the sum of a first and second fraction is always equal to 1: the first fraction is the ratio of the vapor pressure of the product ammonia, in the condenser at the condenser operating temperature, to the absolute pressure in the condenser; the second fraction is the converted value of the percentage of non-condensibles, in relation to the product ammonia in the vapor phase in the condenser. From the above relationship, it will be noted, by those skilled in the art, that as the condensing temperature, for the ammonia, is lowered and as the pressure in the condenser is held constant, the relative percentage of non-condensibles increases in the vapor phase in the condenser.
For example, my calculation indicates that in an ammonia refrigeration system designed for a 95.degree. Fahrenheit (35.degree. Centigrade) cooling medium temperature to the condenser and a 100.degree. Fahrenheit (37.8.degree. Centigrade) temperature for the product ammonia exiting the condenser at a 223 pounds per square inch absolute (15.68 kilograms per square centimeter absolute) pressure within the condenser, the concentration of non-condensibles in the condenser would be about 5%. If the cooling medium were then reduced to 60.degree. Fahrenheit (15.6.degree. Centigrade) and the condensing product ammonia temperature exiting the condenser were then reduced to 65.degree. Fahrenheit (18.3.degree. Centigrade) at the 223 pounds per square inch pressure condition within the condenser, my calculations, for this example, indicate that the concentration of non-condensibles in the condenser would increase to about 47%.
In calculations of this type, it is standard engineering practice to assume a temperature differential of about 5.degree. Fahrenheit (about 2.8.degree. Centigrade), as the temperature of approach. This means that the coldest temperature obtainable by the fluid to be cooled (in the above example, the compressed product ammonia) is about 5.degree. Fahrenheit (about 2.8.degree. Centigrade) more than the temperature of the cooling medium entering the condenser.
My calculations further indicate that if the condenser pressure at the 65.degree. Fahrenheit (18.3.degree. Centigrade) temperature condition for the product ammonia exiting the condenser were reduced to 124 pounds per square inch absolute (8.72 kilograms per square centimeter absolute), the concentration of non-condensibles would be about 5% in the condenser.
The refrigeration system of the ammonia production system also usually includes a compressor which, by compressing the product ammonia being introduced into the condenser, causes the condenser to be pressurized. The calculated effect of the reduction of condenser operating pressure from 223 pounds per square inch absolute (15.68 kilograms per square centimeter absolute) to 124 pounds per square inch absolute (8.72 kilograms per square centimeters absolute) is to reduce the compression power required, by approximately 13%.